Support group renews call in Vienna for end to hostilities and immediate humanitarian access to besieged communities.

World leaders have renewed their calls for a cessation of hostilities in Syria after meeting in Vienna, hoping to restart peace talks aimed at halting Syria's civil war.

The meeting in the Austrian capital on Tuesday was led by John Kerry, US secretary of state; Sergey Lavrov, Russia's foreign minister; and Staffan de Mistura, UN special envoy to Syria, as well as other leaders from the 17-nation International Syria Support Group (ISSG).

They confirmed their support for an end to the violence and the need for immediate humanitarian access to besieged communities inside Syria.

While the parties involved did not set a date for the next round of peace talks, Kerry said the gathering in Vienna was a step in the right direction.

"All the parties ... have agreed on a basic framework which is a united, non-sectarian Syria that is able to choose its future through a transitional governing body," he said.

"The challenge we face now is to transform these possibilites into the reality of an agreement."

However, De Mistura said the involved parties "can't wait too long" to set a date.

Little headway

US officials travelling with Kerry say the US still insists that Syrian President Bashar al-Assad should go as part of a "framework for political transition" with an August 1 deadline.

But Assad, buoyed by military support from Russia and Iran, has shown no sign he is prepared to leave and his forces are still battling for territory.

UN-brokered peace talks in Geneva ended last month without any progress after the opposition delegation walked out, citing increasing attacks by Assad's forces and his Russian ally, as well as problems with humanitarian aid shipments.

In the past weeks, more than 300 people have been killed in government air strikes in Aleppo province alone, while deadly fighting has also raged in other parts of Syria, including Idlib, Deir Az Zor and the outskirts of Damascus.

A truce deal in place since February does not cover armed groups such as the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group and al-Nusra Front, al-Qaeda's Syria branch.

ISIL and al-Nusra Front hold large expanses of the country and carry out attacks.

For their part, government forces continue to blockade several rebel-held areas around Damascus, stopping all food and medical aid in defiance of UN Security Council resolutions.

Mohammad Javad Zarif, Iran's foreign minister, said his country supported the truce and a peace dialogue, but warned that a silencing of arms might aid certain opposition groups operating in Syria.

"We should not allow terrorists to use the cessation of hostilities for further terror operations," he said.

Russia, Assad's key foreign backer, supports the ISSG platform on paper but backs Syrian government forces on the ground.

Russia nevertheless endorsed the UN Security Council resolution that enshrined the ISSG peace plan in international law, and Lavrov says he supports it.

Deadly infighting

On the ground, there has been no apparent let-up in violence. The Syrian Observatory for Human Rights (SOHR) says clashes between armed groups east of Damascus claimed over 50 lives on Monday, bringing to more than 500 the number of people killed in rebel infighting since late April.

The fighting has pitted the Jaish al-Islam group, which is part of the opposition High Negotiations Committee (HNC), against rival faction Failaq al-Rahman.

In a statement on Monday, Jaish al-Islam accused Failaq al-Rahman of rejecting an initiative tabled by Riad Hijab, the HNC coordinator, to end the bloodshed.

The fighting is a challenge to the long dominant position held by Jaish al-Islam in Eastern Ghouta. The group suffered a blow in December when its leader, Zahran Alloush, was killed in an air strike.

The clashes began on April 28, when Jaish al-Islam's positions in Eastern Ghouta came under attack, the UK-based SOHR said.

Failaq al-Rahman has been backed in the fighting by a group known as the Fustat Army, whose fighters include members of al-Nusra Front, the SOHR says.

Elsewhere, fighter jets from the US-led coalition are reported to be continuing their attacks on ISIL targets in Aleppo.

On Tuesday, a Syrian government air strike on Aleppo city left at least three civilians dead, among them a mother and her young daughter, according to SOHR.